FM subcarrier technology has been used in a number of applications, using a variety of analog and digital communication schemes. For example, Muzak, the familiar "elevator" music piped into physicians' offices, elevators and the like, uses a double side band AM modulation of a 67 KHz subcarrier to carry subscription music.
In another FM subcarrier application, known in Europe as the Radio Data System (RDS) and in the U.S. as the Radio Broadcast Data Service (RDBS), a 57 KHz subcarrier is modulated using bi-polar phase shift keying to carry a low speed (1187.5 bps) digital data signal. This technique incorporates a unique block and bit synchronization method as well as a simple linear block encoding for error detection and correction. RDS is a very robust digital subcarrier communication scheme because of its long baud interval (.about.1 ms), low subcarrier frequency, and narrow bandwidth. This technology was originally invented and perfected by the Swedish Telecommunications Office and later extended in the rest of Europe. It has been adopted as an international standard and incorporates specification of the physical layer (the modulation and FM interface), the data link layer (error correction coding), and a network layer for service delivery. The channel modulation efficiency of RDS is about 0.3 bps/Hz.
Because of the low data rate of RDS, another format known as the Data Radio Channel (DARC) was invented by NHK in Japan to support a higher data rate FM subcarrier service. DARC is encapsulated into international standards (cf., EIA-794) as having four modes of operation at the physical level. The differences among these four modes involve the amount of error correction coding (ECC) overhead applied to the data transmission. At the physical layer, DARC is 16K bits per second minimum-shift keyed modulation of a 76K Hz subcarrier tone. DARC specifies all of the first four layers of the communications methodology: Physical, Data Link, Network, and the Transport layers.
Of the four modes of operation, the Frame B mode of DARC provides the best channel coding and error correction ability at the cost of reduced data payload rate. The net data rate, after application of layer 2 and layer 3 overhead, is 6,210 bits per second (bps). DARC offers a channel bit rate efficiency of about 0.66 bps/Hz, the typical value for a minimum-shift keyed modulation. This level of efficiency drops considerably after application of ECC.
Because of DARC's relatively high data rate, it has achieved use worldwide. Several IC manufacturers now deliver highly integrated decoders for receiver/demodulation design. Among the countries actively utilizing DARC systems are Sweden, Germany, Austria, France, Hungary, Japan, and the USA.
Offering comparable performance to DARC is the Subcarrier Traffic Information Channel (STIC) developed by the Mitre Corporation under funding from the Department of Transportation, Federal Highway Administration. This digital system uses a differentially encoded, quadrature phase-shift keyed modulation of either a 72.2K Hz of a 87.4K Hz subcarrier tone to deliver a 18,050 or 21,850 bps raw data rate. STIC also has a US standard (EIA-795) but is little known beyond the USA and has seen virtually no commercial use. Like the above systems, the STIC standard addresses layers 1 through 4 of the communications hierarchy. STIC is notable because it applies modern modem technology to a FM subcarrier system by using efficient convolutional coding, code concatenation and interleaving at the bit level to address channel impairments. The overall efficiency of STIC is on the order of 1.15 bps/Hz at the channel bit rate and a net of about 0.6 bps/Hz. Neither figure represents a very aggressive design. However, STIC was reported to be slightly superior to DARC in terms of overall performance in tests conducted in the USA by the Electronics Industry Association (EIA).
Several other "high-speed" subcarrier technologies have been developed over the past 10 years in the United States. Some of the more notable attempts are Seiko's 19K baud (8K bps nominal), SCA Data System's 32K bps proprietary system, Data Broadcasting Corporation's 19.2K bps FSK system, and Command Audio's proprietary DQPSK system, which is very similar to STIC in concept and structure. Command Audio has a portable subscriber unit, manufactured under license by Thompson Electronics, RCA Consumer Products Division, in commercial trials in Denver and Phoenix at the current time. Again, this system barely reaches a 2 bps/Hz efficiency. By contrast, telephone modem technology operates at 7 bps/Hz almost universally throughout the world, illustrating the difficulty of the propagation environment to which FM subcarrier systems are subjected and the rather low efficiency of the current FM subcarrier systems.
Thus, there is a need in the art for FM subcarrier systems that can achieve higher efficiency and data rates yet utilize the established equipment infrastructure built up by current FM subcarrier standards.